Modulation limiter



H. J. SCHRADER ET AL MODULATION LIMITER Fil ed June 29, 1939 Nov. 30, 1943.

f7ooum TING 21 AMPLIFIER INVE/V'I'ORS:

Harald cl J'Lhrader,

John, M Bru/mbaugh, v 8r Ben W Robina Patented Nov. 30, 1943 UNITED STATES PATENT OF E MODULATION LEIITER Harold J. Schrader, Haddon Heights, and Ben W.

Robins, Haddonlield, N. 3., and John M. Brumbaugh, Lansdowne, 2a., assignors to Radio Corporation of America, a corporation of Delaware Application June 29, 1939, Serial No. 281,930

(01. rte-171.5)

7 Claims:

, Modulated transmitters arev usually provided with a manual control means for reducing the amplitude of the modulating voltage .when it tends to increase beyond a predetermined level. While this manual control system has certain advantages, it is frequently desirable to provide a system which requires no attention and which operates automatically not only to prevent excessive modulation but also to maintain the percent modulation at a substantially constant level notwithstanding fluctuations in the amplitude of the modulating voltage and in the normal transmitter output. The mere provision of an automatic volume control in the audio amplifier of a modulated transmitter does not provide a satisfactory answer to the problem, since normal variations in the sound intensity are thereby reduced, tending to prevent the faithful reproduction of the transmitted intelligence. A biased automatic volume control system in the audio system will prevent overmodulation on occasional peaks, but can not maintain the modunecessitates a readjustment of the amplitude of the modulating voltage, if the percent modulation is to be held at the original level. So also, if a limiting system is used which tends to limit the amplitude of the modulating voltage to prevent overmodulation, the point at which the limiting action is instituted must also be varied when the output of the transmitter is varied. This requires constant attention on the part of an operator, and is therefore objectionable. It is the primary object of this invention, therefore, to provide a modulation system which automatically maintains the percent modulation at a de- I trollable changes in the amplifier plate voltag It is, therefore, a further object of this "inve nected across battery H and secondary 113., A

sired value, or prevents its increase bey'o desired value, independent of changes in transmitter output.

In addition to changing the direct voltage which is applied to the output amplifier fo purpose of readjusting the transmitter outp described above, undesired variations of th rect'voltage source frequently produce This is particularly true in portable transmi ters which are battery operated, or whei transmitter is transferred to an emergencyer supply system. Since the percent are tion is a function of the ratio of the direct p voltage, which is approximately equal to th erage unmodulated carrier voltage on th pi and the superimposed modulating voltage, iting is to be accomplished at a predete" ed percent modulation without continual rea ment to compensate for desired or und' r'e changes in the plate voltage, the point at which,

the limiting action is instituted must varyinpr portion to the amplitude of the plate volt tion to provide a limitingsystem in point of operationis proportional'to the' plate voltage which is appliedto the' modulated in plifier.

This invention will be better understood the following description when considered nection with the accompanying drawing; A figure of the drawing is a schematic diagram of a preferred embodiment of this invention.

Referring to the drawing-,1 is a radio'f quency oscillator of any desired characteris which is coupled to the input of a radio freque amplifier 3. The output circuit of amplifie 3" includes an inductor. 5 and capacitor 1 former of which is coupled to an antenna 9'." plate voltage which is applied to the amii f. is derived from a suitable source such as a n I l which is connected in series with the secdfida' 13 of a modulating transformer I bet ground and the low potential end of induc r A radio frequency by-pass capacitor i5 is con modulating goltage is induced in the second of the modulating transformer by means fa modulating amplifier H, which is in gized by a pair of remote cut-off audio frequency amplifier tubes l9 and2|. A source of modulating voltage 23 is coupled to the grids of the remote cut-off tubes IS'and 2i} by any suitable means, such as a transformer The junction of resistors ll, 43 is connected to the-cathode 45 of a diode rectifier 41 through a parallel circuit comprising a variable resistor 49 and a capacitor SI. Cathode" is also connected to ground through a variable resistor 53. The anode 48. of diode 41 is connected to the unground'ed terminal of biasing resistor 39, and also to ground through a bypass capacitor 55.

The radio frequency amplifier plate voltage is determined by two components, a direct voltage which is produced by the battery II, and an audio frequency voltage which is developed across the secondary I! of the modulation transformer. The modulating voltage is added to the battery voltage during positive peaks of the modulating voltage, and is subtracted from the battery voltage during negative peaks of the modulating voltage. One hundred percent modulation of the radio frequency carrier is therefore accomplished when the amplitude and polarity of the modulating voltage is such that the effective plate voltage on the amplifier tube 3 is reduced to zero. If the modulating voltage exceeds this value deleterious distortion is produced, sincegthe radio frequency output cannot be reduced beyond the zero point. The envelope of the modulated carrier wave, therefore, does not truly represent the wave shape of the modulating voltage.

To prevent this distortion, the amplitude of the modulating voltage must be maintained at a value which is less than the value which reduces the plate voltage to zero. Consequently, variations in the potential of battery ll necessitates the readjustment of the maximum amplitude of the modulating voltage if overmodulation is to be avoided. At the same time, it is to be remembered that it is always desirable to operate the transmitter at the highest average percent modulation possible, in order to fully utilize the available radio frequency energy.

In accordance with this invention, a bias voltage is developed by rectifier 41 when the modulating voltage exceeds a predetermined proportion of the direct plate voltage, which, as noted above, is equivalent to the average unmodulated carrier voltage. It will be seen that thecathode 45 of diode 41 is impressed with a direct voltageand an alternating voltage. The amplitude .of the direct voltage is determined by the attenuation produced in the voltage divider comprising resistors 4| and 43, and the further attenuation which is provided by resistors 49 and 53. A positive D. C. bias is, therefore, applied to cathode 45, the amplitude of which is proportional to the amplitude of the direct voltage which is applied to the amplifier plate. By adjusting either or both of resistors 49 and 53, the amplitude of the biasing voltage may be set to any desired value.

The amplitude of the alternating component is likewise aifected by two attenuators. Resistors 4| and 43 provide a like attenuation for both'the direct and alternating components. The purpose of this attenuator is merely to reduce the alternating and direct voltages which are applied to the diode 41 to usable values. Capacitor 5!, however, bypasses the. audio frequency component around resistor 48,'so that the full alternating voltage across resistor 43 is applied to the oathode. As a result, the proportion of the direct and alternating voltages which are impressed on rectifier 41 may be independently selected, and thus the point at which rectification commences may be adjusted.

When the alternating component present on cathode 48 exceeds the amplitude of the direct component, diode 41 draws a current through resistor II, and the anode '48 develops a negative potential. This negative potential is applied as a bias to the grids of the remote cut-oil ampli-' fiers l9 and 2|. The percent modulation at which the limiting action commences is approximately equal to the attenuation produced by resistors 49 and 53. That is, if the value of resistor 53 is equal to four times the value of resistor 49, then the D. C. component on cathode 45 will be four-fifths of the A. C. component, assuming the two voltages to be of equal amplitude at the midpoint of resistors 4i and 43. The limiting action will, therefore, commence at percent modulation, beyond which the audio amplifiers i9 and 2| receive an increased negative bias to reduce or prevent a further increase in the modulation due to a further increase at voltage at the modulating source 21.

If the output of the transmitter is varied by adjusting the amplitude of the direct voltage produced by battery I I, the direct voltage present on cathode 45 will be likewise altered, so that the limitingaction will automatically commence at that value of modulating voltage which produces the desired maximum modulation at the new plate voltage. The same is true, of course, for variations in the direct plate voltage which occur during normal operation due to changes in the battery or line voltage, since in practice the battery ll would usually be replaced by a rectifierfilter system, as is well known.

A further advantage of the system illustrated arises from the fact that the control bias for the amplifier tubes i9 and 2| is derived from a voltage which is proportional to the actual modulating voltage. Thus, changes in the gain of the modulator H, or in amplifier tubes [9 and 2i, are automatically compensated through the bias voltage developed in the limiting system.

The system illustrated may be operated so as to prevent overmodulation due to occasional peaks by keeping the audio frequency voltage at an average value which is well below that necessary to produce maximum modulation, or if desired the modulation voltage may be increased to a value at which the limiting action is in operatlon a large portion of the time. In the'former case, the original dynamic sound range is maintained, but in the latter case the dynamic range is reduced.

We have thus described an improved system for limiting voltage impulses which would tend to overmodulate a-transmitter, and which may also maintain the average modulation at a selected value. The invention is not to be limited to the specific apparatus illustrated since various modlfications will occur to those skilled in the art. The audio amplifier tubes l9 and 2| may be tri odes, if desired, nor is it necessary to use a pushpull amplifier as shown. It is apparent that other modulating system" may be employed, such as the well known grid modulating system.

We claim as our invention:

1. In a modulation limiter, a radio frequency amplifier, means including a modulation frequency amplifier for applying a modulated direct voltage to said radio frequency amplifier, and

control means operable in response to a modulation frequency voltage whose amplitude exceeds a .predetermined percentage of the applied direct component for deriving a control voltage, means for utilizing said control voltage to limit the gain of said modulation frequency amplifier, said control means including a unilateral discharge device normally biased to cut-oil? solely by adirect voltage proportional to said direct component.

2. In a device of the character described, a thermionic discharge device, a source of direct plate voltage for said device, modulating means including a modulation frequency amplifier for applying a modulating voltage to the plate of said device, a unilateral discharge device, imapedance means in circuit with said unilateral discharge device for applying a control bias to said modulation frequency amplifier, said bias being proportional to the current through said unilateral discharge device, separate means for applying to said unilateral discharge device relatively adjustable direct and alternating voltages proportional to said direct plate voltage and said modulating voltage, respectively, said means comprising a resistive voltage divider across a portion of which a capacitor is connected, the polarities of said voltages being such that said unilateral dischargedevice becomes conductive only when the amplitude of said modulating voltage exceeds a predetermined proportion of the direct plate voltage of said amplifier tube. i

3. In a modulation limiter, a radio frequency amplifier, means including a modulation frequency amplifer for applying a modulated direct voltage to said radio frequency amplifier, a unilateral discharge device having cathode and anode electrodes, means for applying to said device a controllable biasing potential proportional to a desired fraction of the direct component applied to said radio frequency amplifier, said means being the sole biasing means for said device, means for also applying to said device an alternating voltage proportional to the modulation frequency component of said applied voltage to thereby develop a control voltage whenever said modulation frequency component exceeds said controllable biasing potential, and means for applying said control voltage to said modulation frequency amplifier to reduce the gain of said amplifier.

4. In a modulation responsive device for a high frequency amplifier having a cathode electrode and another electrode between which unidirectional operating voltage and modulating signal a portion of the rest of said voltage divider for noneconductive but becomes conductive whenever said signal voltage exceeds a predetermined constant fraction of said operating voltage irrespective of the magnitude thereof.

5. In a modulation limiter for a high frequency amplifier having a cathode electrode and another electrode between which unidirectional operating voltage and modulating signal voltage are supplied, said operating voltage being subject to variations, a voltage divider connected between said electrodes, a unilaterally conducting device connected across a portion of said voltage divider adjacent said cathode electrode and poled to be rendered non-conductive by the unidirectional potential thereon, means to short circuit at least a portion of the rest of said voltage divider for signal current whereby a greater portion of said signal voltage is applied to said unilaterally conductive device than of said unidirectional voltage and said device becomes conductive when the ratio between such portions exceeds a predetermined value unchanged by variations of said operating potential, means operated by current in said device for reducing said signal voltage.

6. In a modulation limiter for a high frequency amplifier having a cathode electrode and another electrode between which unidirectional operating voltage and modulating signal voltage are supplied, said operating voltage being subject to variations, a unilaterally conducting device, electrically passive means for impressing a bias voltage upon said device with such polarity as to render said device non-conductive, said bias voltage being derived entirely from said operating voltage between said electrodes, means to supply signal voltage to said device to render said device conductive when peaks of said signal voltage exceed said operating voltage applied thereto, the ratio of the signal voltage to the operating voltage supplied to said device being greater than the ratio of signal voltage to operating voltage between said electrodes, whereby said device becomes conductive whenever said signal voltage exceeds a predetermined fraction of said operating voltage between said electrodes irrespective of the magnitude of said operating voltage, and means responsive to current in said unilaterally conducting device to reduce said signal voltage.

7. In a modulation responsive device for a high frequency amplifier having a cathode electrode and another electrode between which unidirectional operating voltage and modulating signal voltage are supplied, said operating voltage being subject to variations and the total voltage between said electrodes being subject to undesired reversal upon high peaks of said signal voltage,

' entirely from said operating voltage upon said signal currents so that said device is normally 15 device with such polarity as to render said device I nonconductive so long as peaks of said signal voltage do not exceed said operating voltage, and means including said first means for supplying said voltages to said device with such relative intensities that said device is normally nonconductive but becomes conductive whenever said signal voltage exceeds a predetermined constant fraction less than unity of said operating voltage between said electrodes irrespective of the magnitude thereof.

- HAROLD J. SCHRADER.

BEN W. ROBINS. JOHN M. BRUMBAUGH.

and current responsive 

